This invention is in the field of treatment of strings for sports equipment or musical instruments to reduce their coefficients of friction.
In the case of sports equipment such as tennis racquets, easy relative movement of strings on surfaces, in guides, grooves, sheaves, and eyes, and against other strings is often desirable. Low friction facilitates stringing of the racquet and reduces wear and abrasion during use, and improves performance by allowing the smooth stretching and contraction of the strings under impact. For example, tennis racquets function better if the strings move smoothly and easily relative to one another. This increases the time the ball is in contact with the racquet, reduces the shock of impact, and improves the control the player exercises over the direction and the spin of the ball. Wear is reduced and string life increased. Similarly, other stringed devices, such as musical instruments and fishing rods and reels, are more easily assembled, adjusted, and used when the strings move smoothly and with minimum friction against each other and in the various guides, supports, sheaves, and eyes that characterize the devices.
A number of ways have been proposed to modify strings to reduce their coefficients of friction and facilitate their movement. These include application of lubricants (U.S. Pat. No. 5,863,298), coating with dispersions of particles of TEFLON(copyright) resin (U.S. Pat. No. 4,377,620; Japanese Patent no. 4-80149), coating with molten polyamide or polyester containing particles of TEFLON(copyright) resin (U.S. Pat. No. 4,377,620), and melt-extruding fluoropolymer resin onto strings (European Patent no. 257424). Lubricants though slippery, are soft and fugitive, picking up dust and dirt. Lubricants also have a tendency to transfer to whatever comes in contact with the lubricated string. Particles of TEFLON(copyright) resin are hard, do not adhere well to any substrate, and can be made to melt, flow, and coalesce, if at all, only at temperatures near or above the melting points of polymers used in making racquet strings, such as nylon. Using TEFLON(copyright) resin particles in binders such as polyamides requires high temperature for application, and the resulting coating, being composed of TEFLON(copyright) resin and binder, does not have the low coefficient of friction of the fluoropolymer used alone. Extruded coatings of fluoropolymers have good friction properties, but even higher temperatures are necessary in melt extrusion, 300 to 400xc2x0 C., and because of the viscosity of the molten polymer, the coating thickness is on the order of 25 xcexcm or greater. The high temperature limit the materials of which the string can be made, and the thickness of the coating is a disadvantage, affecting the properties of the coated string, and requiring the use of a large mass of relatively expensive fluoropolymer when only the beneficial surface property, that is, the low coefficient of friction of the fluoropolymer, is needed. Furthermore, the fluoropolymer coatings do not adhere well to the string.
Low coefficient of friction is beneficial also for the interior surfaces of composite strings, that is, strings that are made up of smaller strings, or filaments or fibers. Such strings benefit if the components can move easily with respect to one another when forces are applied, such as by impact. Tennis strings are often composed of a central monofilament or multifilament strand around which are wound or braided smaller mono- or multifilament strands, with a jacket around the whole. Elasticity is improved if there is little or no friction within the strings themselves. U.S. Pat. No. 4,382,358 describes a string comprising a monofilament of a copolymer of polyvinylidene fluoride. Japanese Patent no. 6-218081 describes a composite string as a core-sheath structure, comprising a nylon core and a sheath in which at least some of the filaments are made of polymer containing fluorine. These patents teach the use of fluoropolymers as components of the composite string. However, this requires excessive amounts of fluoropolymer in the case where only a surface property, that is the low coefficient of friction, of fluoropolymer is wanted.
There is a need for very thin, adherent, easily applied coatings having low coefficients of friction for strings in sports racquets and other stringed devices, such as musical instruments and fishing rods and reels. Furthermore, there is a need for treatments that permit the component strands of composite strings to move easily with respect to one another, that is, to reduce friction at the interior surfaces of composite strings.
An object of the present invention is to provide strings for stringed devices, such as sports racquets and musical instruments, having low coefficients of friction in order to optimize operation and lifetime of the strings in the stringed devices.
In one embodiment, the present invention is directed to a coated string comprising: (a) an first coating of fluoropolymer having recurring units containing polar functional groups coated on the string; and (b) a second coating of fluoropolymer having recurring units containing no polar functional groups surrounding the first coating.
In another embodiment, the present invention is directed to a sports racquet strung with a coated string comprising: (a) an first coating of fluoropolymer having recurring units containing polar functional groups coated on the string; and (b) a second coating of fluoropolymer having recurring units containing no polar functional groups surrounding the first coating.
In another embodiment, the present invention is directed to a composite string having component strands, wherein said component strands are coated with fluoropolymer coating. In a preferred embodiment, the composite string comprises a first coating of fluoropolymer having recurring units containing polar functional groups and a second coating of fluoropolymer having recurring units containing no polar functional groups surrounding the first coating.
In another embodiment, the present invention is directed to a process for coating string, comprising applying a first solution of a first fluoropolymer containing recurring units having polar functional groups to a string to form a coated string, drying said first solution, applying a second solution of a second fluoropolymer containing recurring units having no polar functional groups to the coated string and drying said second solution.
In another embodiment, the present invention is directed to a composite string comprising a multitude of strands, wherein the surfaces of said strands are interspersed with particles of fluoropolymer.